This invention relates to a process for controlling the slip when braking a road vehicle, and more particularly, for improving control in case of different adhesive or friction values on the right and left sides of the vehicle. The rotational behavior of the vehicle's wheels and the vehicular velocity or derived measured values are detected. The corresponding signals are logically combined as well as processed and, upon the instabilization of a wheel such as upon the occurrence of a lock-up tendency, the braking force at the respective wheel is kept constant or reduced and is thereafter reincreased. Circuit arrangements for carrying out the process are provided by the present invention.
Many variations of processes and circuits of this type are known. By means thereof, the rotational behavior of individual wheels or of wheels combined in groups is measured during the braking operation. After the evaluation, such as after the processing and logical combination of the measured values while considering certain marginal conditions, there is an intervention in the braking pressure variation if lock-up tendencies are registered at any one or at several of the wheels due to an excessive braking force or due to an excessive braking pressure. Even under unfavorable conditions, in particular on slippery road surfaces, the brake slip is controlled to a value favorable for the braking behavior of the vehicle in order to prevent the dangers accompanying a wheel lock-up. That is, the danger of skidding and the loss of steerability of the vehicle while attempting to achieve a short stopping distance.
Particular difficulties are encountered in rating control for road conditions where the friction values between the tires and the road surface vary widely between the right and left vehicle sides. For example, these situations prevail in the wintertime when the road surface is dry on the one side while localized accumulations of snow or ice can be found. Further, it is likely that by swerving around a critical situation that the vehicle has one of its sides on an unsurfaced road with a very bad adhesive value. If, in such situations, slip control rates the braking pressure for the two wheels of one axle in accordance with the vehicle side with the low adhesive value, there will be an excessive reduction of the braking pressure and the stopping distance thus will become excessive. In case of individual control of the two wheels of one axle which are running with very different adhesive values there will result a strong braking effect on the vehicle side with a good grip, and only on this side. Thus the braking effect will result in a high yawing moment which often cannot be compensated by the steering corrections of the driver. A slip control rated for a short stopping distance thus endangers the driving stability of the vehicle in such situations.
In order to reduce this effect it has been suggested to install a pressure limiting device in the pressure medium supply system of a slip-controlled brake system. The pressure limiting device directly compares the controlled braking pressures of the two wheels of one axle and influences the higher controlled braking pressure when a certain pressure difference is exceeded (German Patent Specification=DE-PS No. 23 33 127). It is disadvantageous in such an anti-skid control system that the pressure difference limitation is effective and lengthens the stopping distance even when, due to the road conditions, the controlled pressure, which is not limited, would not lead to a yawing moment or to only a small yawing moment. On the other hand, in unfavorable situations, the rigidly predetermined response pressure difference may not be sufficient to prevent the yawing moment as in this case the condition of the wheel brakes may result in erroneous friction value, friction radius, etc.
It is, therefore, an object of this invention to develop a process for controlling the brake slip which, in all practical situations, prevents the formation of a yawing moment dangerous to the vehicle and which at the same time ensures a high adherence ability of the wheel running with a high friction value and thus enables a short stopping distance to be achieved.